diff --git a/src/SmtpServer.Tests/SmtpParserTests.cs b/src/SmtpServer.Tests/SmtpParserTests.cs
index 555093c..08056dd 100644
--- a/src/SmtpServer.Tests/SmtpParserTests.cs
+++ b/src/SmtpServer.Tests/SmtpParserTests.cs
@@ -312,6 +312,24 @@ public void CanMakeProxyTcp6()
             Assert.Equal(16789, ((ProxyCommand)command).DestinationEndpoint.Port);
         }
 
+        [Theory]
+        [InlineData("PROXY TCP5 192.168.1.1 192.168.1.2 1234 16789")]
+        [InlineData("PROXY TCP46 192.168.1.1 192.168.1.2 1234 16789")]
+        [InlineData("PROXY TCPA 192.168.1.1 192.168.1.2 1234 16789")]
+        public void CanNotMakeProxyWithInvalidTcpVersion(string input)
+        {
+            // arrange
+            var reader = CreateReader(input);
+
+            // act
+            var result = Parser.TryMakeProxy(ref reader, out var command, out var errorResponse);
+
+            // assert
+            Assert.False(result);
+            Assert.Null(command);
+            Assert.Null(errorResponse);
+        }
+
         [Fact]
         public void CanMakeAtom()
         {
diff --git a/src/SmtpServer/Protocol/SmtpParser.cs b/src/SmtpServer/Protocol/SmtpParser.cs
index 3b57b32..857eb84 100644
--- a/src/SmtpServer/Protocol/SmtpParser.cs
+++ b/src/SmtpServer/Protocol/SmtpParser.cs
@@ -845,7 +845,7 @@ public bool TryMakeTcp4Proxy(ref TokenReader reader, out SmtpCommand command)
             }
 
             var token = reader.Take();
-            if (token.Kind != TokenKind.Number && token.Text[0] != '4')
+            if (token.Kind != TokenKind.Number || token.Text.Length != 1 || token.Text[0] != '4')
             {
                 return false;
             }
@@ -869,7 +869,7 @@ public bool TryMakeTcp6Proxy(ref TokenReader reader, out SmtpCommand command)
             }
 
             var token = reader.Take();
-            if (token.Kind != TokenKind.Number && token.Text[0] != '6')
+            if (token.Kind != TokenKind.Number || token.Text.Length != 1 || token.Text[0] != '6')
             {
                 return false;
             }